How to solve: Notice: Undefined Index

Happens when you try to access an array by a key that does not exist in the array.

A typical example for an Undefined Index notice would be (demo)

$data = array('foo' => '42', 'bar');
echo $data['spinach'];
echo $data[1];

Both spinach and 1 do not exist in the array, causing an E_NOTICE to be triggered.

The solution is to make sure the index or offset does exist, either by using array_key_exists or isset prior to accessing that index.

$data = array('foo' => '42', 'bar');
if (array_key_exists('spinach', $data)) {
    echo $data['spinach'];
}
else {
    echo 'No key spinach in array';
}

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How to solve: Fatal error: Using $this when not in object context

$this is a special variable in PHP which can not be assigned. If it is accessed in a context where it does not exist, this fatal error is given.

This error can occur:

1. If a non-static method is called statically. Example:

   class Foo {
      protected $var;
      public function __construct($var) {
          $this->var = $var;
      }
   
      public static function bar () {
          // ^^^^^^
          echo $this->var;
          //   ^^^^^
      }
   }
   
   Foo::bar();

   How to fix: review your code again, $this could only used in object context, and should never in a static method. And also, static method should not access the non-static property. Use self::$static_property to access the static property.
2. If code from a class method has been copied over into a normal function or just the global scopeand keeping the $this special variable.
   How to fix: Review the code and replace $this with a different substitution variable.

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How to solve: Parse error: syntax error, unexpected T_XXX

This happens when you have T_XXX token in unexpected place, unbalanced (superfluous) parentheses, use of short tag without activating it in php.ini, and many more.

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How to solve: Parse error: syntax error, unexpected '['

This error comes in two variatians:

Variation 1

$arr = [1, 2, 3];

This array initializer syntax was only introduced in PHP 5.4; it will raise a parser error on versions before that. If possible, upgrade your installation or use the old syntax:

$arr = array(1, 2, 3);

See also this example from the manual.

Variation 2

$suffix = explode(',', 'foo,bar')[1];

Array dereferencing function results was also introduced in PHP 5.4. If it's not possible to upgrade you need to use a temporary variable:

$parts = explode(',', 'foo,bar');
$suffix = $parts[1];

See also this example from the manual.

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